Filtration methods of treatment of wastewaters are presently performed by a number of different methods, which include chemical methods and mechanical methods. Mechanical methods of filtration typically operate by physical exclusion; the wastewater is passed through a porous medium, which blocks particles larger than the size of the pores but allows the transit of water and particles smaller than the pores (the filtrate). Decreasing the size of the pores improves the quality of the filtrate permeating through the porous medium, but typically requires more powerful pumps and higher energy expenditures to counter the pressure drop created by the smaller pore size. Mechanical methods of filtration fall into two general categories: cross-flow and through-flow filtration. In through-flow filtration, both wastewater flow and filtrate flow are normal to the surface of the filter medium; thus the filtered particles continuously accumulate on and within it. The filtrate flux steadily decreases with time when the pressure drop across the filter is maintained constant, and frequent "back-washing" is necessary to remove the accumulated solids from the filter matrix.
In cross-flow filtration, the wastewater flow is mostly parallel to the filter surface, with the filtrate permeation occurring perpendicular to the flow. A quasi-steady operation is possible, because the continuous build-up of the separated solids on the filter surface is largely prevented by the hydrodynamic shear exerted by the cross-flow. Consequently, a major advantage of cross-flow filtration over through-flow filtration is its high filtration rate.
Cross-flow filtration technology can be separated into two different branches, microfiltration and ultrafiltration. Microfiltration removes primarily suspended solids, and requires relatively low filtration pressures (around 15 psi); it can use relatively rugged thick-walled porous tubes, such as the HYDROPERM.TM. microfiltration tubes manufactured by DYNAFLOW, INC. of Fulton, Md., whose pore structure and distribution in the thermoplastic polymer material can be controlled during the manufacturing process. Ultrafiltration, on the other hand, intends to remove substances at the molecular level in addition to suspended solids, requires higher pressures (around 50 psi), and employs relatively fragile thin membranes. The present invention is more specifically targeted at the microfiltration field, and is well suited for the optimal use of HYDROPERM.TM. microfiltration tubes. Systems applying the teachings of the present invention in combination with these microfiltration tubes are called DYNAPERM.TM. systems.
Swirling the flow in a chamber containing the porous filter element coaxially at the center of the generated vortex presents advantages in the filtering of water containing particles in suspension. Among these advantages is the fact that the particles are being moved to the periphery of the chamber in which the vortex takes place, whereas the filter element is located at the center of the chamber. Therefore, the particles are prevented from clogging the surface of the porous filter element and from increasing the pressure drop across the filter. Another advantage is the fact that an effective cross-flow filtration regime is achieved, in which the shear of the liquid flow along the surface of the filter element prevents the excessive build up of the layer of particles on this filter element, thereby maintaining the pressure drop to an acceptable level. In the prior art these advantages have not been exploited to their full potential. Indeed in the prior art the geometries of the components generating the swirl are not inductive to producing flow conditions that are similar and optimal along the whole length of the filter element. For example, U.S. Pat. No. 4,597,871 to Okouchi discloses a cyclone configuration for the removal of small marine organisms from seawater to be used as a cooling liquid for power plants. The inlet is a single pipe located at the top of the chamber, and the filter element extends much below the inlet. The parts of the filter element close to the inlet will be subjected to much stronger vortex action than the parts away from the inlet. This inhomogeneous flow configuration may also lead to regions of idle liquid where the particles in suspension are allowed to drop by gravity and accumulate at the bottom of the chamber. The present invention is distinct from the teachings of Okouchi. In the present invention the swirling action is generated by long narrow slits that extend mostly all the way along the falter element, with the goals that regions of idle fluid are avoided and that along its whole length the filter element is subjected to similar and optimal vortex flow conditions. Okouchi does not teach this type of configuration.